The present invention relates to a process for the air classification of a grain or particle mixture with reclassifying as well as an air classifier.
Such a process and a corresponding air classifier are known from publication EP 23 320 Al. For separating the fine material particles or fines from the classifying air, this known air classifier generally requires dust separators, e.g. in the form of cyclone separators and filters. The throughput of fines of such a known air classifier is more particularly dependent on the vertical height and the diameter of its centrifuge basket, i.e. its circumferential surface or the cylindrical annular chamber in which classification is performed. The other essential parameter of a classification process, namely the particle size limit differentiating the fine material from the coarse material, is on the one hand determined by the diameter and speed of the centrifuge basket and on the other is dependent on the external diameter of the classifying chamber and the inflow rate of the classifying air into the classifying chamber. In both cases the particle size limit is dependent on the accelerative forces acting on the material being classified.
It is theoretically possible to increase the diameter of the centrifuge basket. However, practical restrictions occur due to the centrifugal forces increasing on a square law basis and the high compressive forces acting on the centrifuge basket ledges. Another aspect acting against the increase in the size of the centrifuge basket diameter is the decrease in curvature of the classifying air paths as the diameter increases. This can admittedly be compensated by higher outflow speed or higher classifying air quantities, but these and the increasing resistance losses would require a higher energy expenditure.
Limits are also placed with regards to the vertical dimensioning of the centrifuge basket due to the mechanical loading of its peripheral ledges. However, an increase in the vertical size of the centrifuge basket is particularly limited due to the occurrence of different flow rates along the basket ledges when classifying air and fines pass between said ledges.
Thus, also in this air classifier, the flow rates along the jacket-like circumferential surface of the centrifuge basket is essentially directly dependent on the suction within the basket. This suction is at a maximum level at the fines outlet from the centrifuge basket and decreases towards the facing, closed end face of said basket. The rotary drive by means of the vertical drive shaft takes place by means of said closed or at least substantially closed end surface in the known air classifier.
These different flow rates when the fines enter the centrifuge basket at different vertical heights thereof lead to differences in the separation quality or selectivity. It is clear that in the vicinity of the classifying air outlet where the maximum suction action occurs, oversize material passes into the fines, whilst at the greatest distance from the classifying air outlet undersize material remains in the coarse material. These serious selectivity disadvantages increase with the magnitude of the vertical extension of the centrifuge basket. However, these disadvantages would appear to be unavoidable in the case of air classifiers with a high throughput capacity. In the case of the air classifier according to EP 23 320 Al, to the aforementioned disadvantage is added that, considered circumferentially, different flow rates would preponderate on the centrifuge basket, because at this point by means of differently designed spiral supply ducts in an approximately tangential arrangement with respect to the centrifuge basket, classifying air quantities of the most varied types, e.g. dust-laden, hot mill spent air and opposite to this, e.g. cooler, but dust-free, atmospheric air are blown into the air classifier.
More particularly in order to overcome the latter disadvantages of different flow rates in the vertical direction along the centrifuge basket, in another known air classifier according to European patent 67 895B1 classifying air outlets are provided on both end faces of the centrifuge basket. However, the problem then occurs that as a result of the considerable vertical extension compared with the centrifuge basket diameter, it is necessary to support and reinforce the basket ledges by a plurality of approximately radially positioned spokes. This is necessary so that it is sufficiently reliably possible to counteract the effects of the operationally occuring centrifugal forces and the other mechanical stresses, such as vibrations. These reinforcing spokes, however, come into contact with the fine material particles contained in the classifying spent air and are therefore subject to relatively high wear. It can be assumed that this known air classifier has been developed for less abrasive classifying material, as is apparent from the upper particle size limit given for it of 2 to 4 .mu.m, in order to obviate this wear problem. As a result of these particle sizes the classifying air removed contains much lower solids concentrations to which the radial spokes are exposed.
Therefore the last-mentioned, known air classifier would not appear to be suitable for a highly abrasive classifying material, such as sintered alumina, quartz sand or cement clinker in the case of solids concentrations of e.g. 800 to 1200 g/m.sup.3 in the classifier spent air. Particularly when the material to be classified is a particle mixture having serious abrading characteristics, it is scarcely possible to consider continuous operation over several weeks or months without prolonged shutdowns for repairs or renewal of the centrifuge basket.
On the basis of this prior art, the problem of the present invention is to so design an air classifying process and air classifier for performing it, that the aforementioned disadvantages are avoided and that even in the case of relatively abrasive classifying material a long operating period, in the case of high throughput and improved separation efficiency are achieved, whilst optionally achieving this with a relatively small installation surface for the air classifier.